Autonomic nervous system (ANS) is the ‘unconscious’ nervous system that controls and regulates virtually all of our basic body functions, such as cardiac function, blood circulation and glandural secretion. The main parts of the ANS are the parasympathetical and sympathetical nervous branches. The sympathetical nervous system usually prepares us for high stress situations by speeding up body functions, while the parasympathetical system restores, under conditions of normal ANS regulation, normal conditions in blood circulation by slowing down the heart rate (HR). The heart rate is mainly controlled by the parasympathetical vagal nerve. Pain, discomfort, and surgical stress may activate the sympathetical branch of the ANS and cause an increase in blood pressure, heart rate and adrenal secretion.
Sympathetical activation is often manifested in large low frequency (LF) variations in the heart rate, in blood pressure, and peripheral blood circulation. Vagal activation is mainly seen in heart rate, but also in blood pressure and circulation in high frequency (HF) band, in which modulations are usually largest. The HF component arises mainly due to respiratory influence. The sympatho-vagal balance is described by the LF/HF power ratio. This ratio is traditionally estimated in spectral domain. Fourier analysis is used to calculate the spectral power at fixed LF (below 0.15 Hz) and HF (from 0.15 to 0.4 Hz) frequency bands. The technique is well known in Heart Rate Variability (HRV) analysis.
Pain is an unpleasant sensory or emotional experience that is associated with actual or potential tissue damaging stimuli. It is always an individual and subjective sensation, which may be acute (nociceptive), elicited by noxious stimuli, or chronic pain that has outlived its usefulness to preserve tissue integrity. The perception of pain takes mainly place at cortex, and it may be suppressed in deep sedation and anesthesia by the general (global) inhibitory effects of sedative drugs and anesthetic agents. The responses to noxious stimulus may also be suppressed when the pain signal pathway is sufficiently suppressed at the subcortical level, often in the region of the brainstem and spinal cord. Both cortical and subcortical mechanisms play a role in pain management in modern surgical anesthesia or intensive care.
Analgesia refers to the absence of pain or loss of sensitivity to pain without unconsciousness in response to stimulation that would normally be painful.
When developing ‘index type’ numeric or other indicators reflecting the state of a patient, such as the activity of the ANS, the basic difficulty is to associate the index with a fixed scale in situations, in which the basic physiological parameters, such as the HR, measured from the patient do not have any ‘normal’ values, but vary over a wide range of values even in case of healthy patients. A special difficulty when evaluating the state of the ANS with the objective of getting an estimate of the adequacy of analgesia, for example, is the lack of an exact measure of the adequacy of analgesia, i.e. there is no quantity that can be directly related either to the adequacy of analgesia or to a specific drug (opioid) effect or body reflex. Furthermore, a change in a basic physiological parameter measured may indicate another physiological cause than the (in)adequacy of analgesia. In other words, the difficulty also lies in finding a measure that would be specific to the variable estimated, such as to the adequacy of analgesia.
Artificial ventilation of a patient shall often be considered as a stress factor for the patient. It can also generate artifacts in the signal, because the HF modulation may be excessively influenced by the resulting overpressure in the lungs and airways. In spontaneous (normal) breathing, this situation is seldom reached in such a degree. In normal breathing, the pressure and flow sensitive receptors in the atria of the heart and in the pulmonary and aortic vessels signal differently than in artificial overpressure ventilation. For example, the heart rate of a spontaneously breathing patient accelerates during inhalation and decelerates during exhalation, whereas the opposite occurs in overpressure ventilation. The ANS regulation of the blood circulation and heart rate is thus disturbed, which calls for special algorithms for estimating the sympathetical and parasympathetical activations and their balance.
As mentioned above, the sympatho-vagal balance is a well-known tool in HRV analysis for examining cardiovascular neural regulation. A general drawback related to the determination of the sympatho-vagal balance is that the current analysis method based on Fast Fourier Transform (FFT) is not suitable for real-time monitoring of a patient. This is due to the fact that a certain time, typically at least 1 to 2 minutes, is needed to obtain the frequency components of the signal. The main reason for the delay is the time needed to analyze the low frequency variations, i.e. the LF component of the signal, since several cycles are needed for the result.
Furthermore, the current analysis method is not suitable for patient monitoring systems requiring an analysis of non-stationary signals, such as noxious responses. Fast responses, i.e. responses with durations of about 10 to 15 sec, always cause non-stationarities in the signal. The FFT does not yield a reliable result in case of non-stationary signals including step-like changes in the signal values, and therefore the FFT should not be used for such signals.
The present invention seeks to alleviate or eliminate the above drawbacks and to bring about a mechanism that enables reliable real-time monitoring of the state or activity of the autonomous nervous system of a patient.